1. Field of the Invention
The present invention relates generally to the separation of gold and silver from ores. More particularly, the present invention relates to the separation of gold and silver from ores by the method of heap leaching in which the precious metal values are recovered as cyanide complexes which are then separated from solution by adsorption on activated carbon. Still more particularly, the present invention relates to an improved method of carbon desorption in which metal cyanide complexes are removed from the activated carbon.
2. Description of the Prior Art
The recovery of gold and silver by the process of heap-leaching is well known. In general, the procedure comprises spraying, trickling, pouring on, or otherwise applying to a pile of low grade ore or tailings an aqueous alkali cyanide solution, e.g., aqueous calcium, sodium or potassium cyanide, so as to cause the solution to permeate and percolate through the pile thereby extracting the gold and silver as cyanide complexes. The resultant cyanide complex pregnant liquor is recovered from the bottom of the pile, of which there is usually a series in an extraction facility, and routed to a recovery system. A recent improvement designed to increase the efficacy of the heap-leach procedure comprising the incorporation in the leach solution of any of various hydrolyzable surfactants has been disclosed in U.S. Pat. No. 4,929,274 of Lionel B. Luttinger, one of the inventors of the invention of this application.
The cyanide complexes obtained from the ore by heap-leaching are separated from the pregnant liquor by adsorption on activated carbon in pulp (CIP) or activated carbon in column (CIC), after which the resultant loaded carbon is subjected to stripping. Stripping is the process of carbon desorption by which the metal cyanide complexes are put into solution. The gold and silver values then being recovered from the solution by electrowinning or by zinc precipitation.
Two types of strip circuits are presently employed. One is known as a "Zadra" strip and the other as an "AARL" strip. In both types, an aqueous barren strip solution of sodium cyanide and caustic soda, heated to 180.degree.-300.degree. F., is passed through the loaded carbon in a strip column or vessel. The heated solution desorbs or "strips" the gold and silver cyanide complexes from the carbon which are then recovered from solution as above discussed. In the "Zadra" strip, the strip solution is continuously recirculated until the maximum or optimum recovery of gold and silver from the carbon is effected. In the "AARL" strip, the strip solution is not recirculated. This latter described strip therefore, is a once-through process.
Sodium cyanide is unstable in caustic solution at elevated temperatures and breaks down to form ammonia gas. This reaction necessitates a constant makeup of cyanide to the barren solution of a strip circuit in order to maintain its efficiency. Approximately 20% of the cyanide expense of a typical strip circuit is the result of cyanide loss due to its instability which is of particular significance since cyanide is currently in short supply. Sodium cyanide, moreover, is a deadly environmental hazard.
It has been proposed to modify the caustic-cyanide strip solution by eliminating the cyanide from the solution. In current practice, however, the metal complex-bearing carbon is preferably first subjected to an acid pre-soak step in order to remove calcium carbonate and other substances from the carbon pores. Because acid presoak followed by contacting with a non-cyanide strip solution can irreversibly fix much of the gold and silver content on the carbon with no possibility of stripping except with a caustic-cyanide solution, the use of a cyanide-free strip solution has not been considered feasible so long as a pre-soak is practiced.